The present invention relates to radio frequency (RF) measurements, and more particularly to a dual channel measurement system for simultaneous adjacent channel leakage ratio (ACLR) measurement.
Modern RF measurement equipment typically converts an input RF signal to an intermediate frequency (IF) signal which is then converted into a digital domain by an analog-digital converter (ADC). The intricate signal processing and measurement functions, such as applying a final measurement bandwidth to the signal, demodulation of the signal with subsequent modulation measurement if desired, or perhaps detection and logarithmic compression of the signal to obtain a spectral amplitude display, are then performed in a digital signal processing (DSP) system. The major difficulty of such an approach is with the ADCs. An ADC suitable for very wide dynamic range display of spectral components featuring a large number of output bits of precision may have a limited bandwidth, making demodulation of wideband signals impossible. An ADC suitable for measuring wideband signals featuring a very high conversion rate has a limited number of output bits, making wide dynamic range measurements impossible.
The difficulty of the selection of ADCs is highlighted by the desire of those in the wireless communications industry to make what is known as a simultaneous ACLR measurement, which is the ratio of the power within a transmitter's occupied channel to power leaked by the transmitter into an adjacent channel. When performing simultaneous ACLR measurements five or more wideband wireless signals are present. The term “simultaneous” means that measurements must be made simultaneously of each carrier's power and the resulting adjacent channel leakage signal. The reason for this requirement is that the instantaneous power of each channel varies somewhat, and it is desired to correlate the amplitude of the leakage signal with the variations in individual carrier power. This cannot be done if each signal is measured sequentially.
One approach to this measurement is to use a very high conversion rate ADC with as many bits of precision as possible. This limits the dynamic range of the ACLR measurement below what the wireless communications industry desires, so there is intense pressure to improve measurement performance. Another approach is to use three state of the art ADCs, each measuring a sub-band of the incoming signal. The results are “stitched” together using very complicated DSP techniques involving error correction in each channel, taking a fast Fourier transform (FFT) of each channel, stitching together the spectrum of the three FFTs, and then taking the inverse FFT of the result to achieve the time record of the wide measurement channel. This approach requires three very fast, state-of-the-art ADCs with a large number of bits and a large DSP processor to achieve the wideband result, but still does not achieve quite as much dynamic range as desired.
What is desired is a system for measuring simultaneous ACLR that is more accurate than present techniques.